Ultrafast graphene photodetector unveiled

By Edd Gent

Published Wednesday, April 15, 2015

A new ultrafast graphene-based photodetector could drastically boost operation speeds in optical technologies.

The device developed at the Institute for Photonic Sciences (ICFO) in Barcelona is capable of converting incident light into electrical signals on femtosecond timescales, three orders of magnitude quicker than lasers currently used in optical communications and medical imaging.

Converting light into electricity underpins a range of technologies including solar cells, digital cameras and optical-fibre communications and in most cases operation speed is critical.

“Graphene photodetectors show fascinating performance and properties, enabling a wide range of applications,” said professor Frank Koppens, whose research group is affiliated with the EU’s Graphene Flagship project.

“Ranging from multi-spectral imaging to ultra-fast communications, such applications are being actively developed within the Graphene Flagship programme.”

Graphene has long been considered a promising material for ultrafast, broadband photodetectors, but previous efforts have been limited to picosecond detection times by slower switching rates.

These photodetectors generate photovoltage via the photo-thermoelectric effect that occurs when incident light is focused at the interface between graphene layers doped with different chemicals where it heats up charge-carrying electrons.

However, previous approaches have been limited to switching rates of a few hundred gigahertz and scientists knew that achieving the much faster terahertz switching necessary for femtosecond detection would require more efficient charge-carrier heating.

In a paper in journal Nature Nanotechnology, the group describes how they were able to boost the effectiveness of this process with energy from incident photons transferred efficiently to charge-carrier heat with a constant spectral response between visible and infrared wavelengths of 500 and 1,500 nanometres.

This is consistent with efficient electron heating and allowed the researchers to directly measure the duration of a laser pulse less than 50 femtoseconds in length.